.TH std::variant::emplace 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::variant::emplace \- std::variant::emplace

.SH Synopsis
                                                        (since
   template <class T, class... Args>                    C++17)
   T& emplace(Args&&... args);                          (until
                                                        C++20)
   template <class T, class... Args>                    (since
   constexpr T& emplace(Args&&... args);                C++20)
   template <class T, class U, class... Args>                   (since
   T& emplace( std::initializer_list<U> il,                     C++17)
   Args&&... args );                                            (until
                                                                C++20)
   template <class T, class U, class... Args>
   constexpr T& emplace(                                        (since
   std::initializer_list<U> il, Args&&... args                  C++20)
   );
   template <std::size_t I, class... Args>                              (since
   std::variant_alternative_t<I, variant>&                              C++17)
   emplace( Args&&... args );                                           (until
                                                \fB(1)\fP                     C++20)
   template <std::size_t I, class... Args>                              (since
   constexpr std::variant_alternative_t<I,                              C++20)
   variant>& emplace( Args&&... args );             \fB(2)\fP
   template <std::size_t I, class U, class...
   Args>                                                                        (since
                                                                                C++17)
   std::variant_alternative_t<I, variant>&              \fB(3)\fP                     (until
                                                                                C++20)
       emplace( std::initializer_list<U> il,
   Args&&... args );
   template <std::size_t I, class U, class...                   \fB(4)\fP
   Args>

   constexpr std::variant_alternative_t<I,                                      (since
   variant>&                                                                    C++20)

       emplace( std::initializer_list<U> il,
   Args&&... args );

   Creates a new value in-place, in an existing variant object

   1) Equivalent to emplace<I>(std::forward<Args>(args)...), where I is the zero-based
   index of T in Types....
     * This overload participates in overload resolution only if
       std::is_constructible_v<T, Args...> is true, and T occurs exactly once in
       Types...
   2) Equivalent to emplace<I>(il, std::forward<Args>(args)...), where I is the
   zero-based index of T in Types....
     * This overload participates in overload resolution only if
       std::is_constructible_v<T, std::initializer_list<U>&, Args...> is true, and T
       occurs exactly once in Types...
   3) First, destroys the currently contained value (if any). Then direct-initializes
   the contained value as if constructing a value of type T_I with the arguments
   std::forward<Args>(args).... If an exception is thrown, *this may become
   valueless_by_exception.
     * This overload participates in overload resolution only if
       std::is_constructible_v<T_I, Args...> is true.
     * It is a compile-time error if I is not less than sizeof...(Types).
   4) First, destroys the currently contained value (if any). Then direct-initializes
   the contained value as if constructing a value of type T_I with the arguments il,
   std::forward<Args>(args).... If an exception is thrown, *this may become
   valueless_by_exception.
     * This overload participates in overload resolution only if
       std::is_constructible_v<T_I, std::initializer_list<U>&, Args...> is true.
     * It is a compile-time error if I is not less than sizeof...(Types).

.SH Parameters

   args - constructor arguments to use when constructing the new value
   il   - initializer_list argument to use when constructing the new value

.SH Return value

   A reference to the new contained value.

.SH Exceptions

   1-4) Any exception thrown during the initialization of the contained value.

.SH Example


// Run this code

 #include <iostream>
 #include <string>
 #include <variant>

 int main()
 {
     std::variant<std::string> v1;
     v1.emplace<0>("abc"); // OK
     std::cout << std::get<0>(v1) << '\\n';
     v1.emplace<std::string>("def"); // OK
     std::cout << std::get<0>(v1) << '\\n';

     std::variant<std::string, std::string> v2;
     v2.emplace<1>("ghi"); // OK
     std::cout << std::get<1>(v2) << '\\n';
     // v2.emplace<std::string>("abc"); -> Error
 }

.SH Output:

 abc
 def
 ghi

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to              Behavior as published               Correct behavior
   P2231R1 C++20      emplace was not constexpr while the required     made constexpr
                      operations can be constexpr in C++20

.SH See also

   operator= assigns a variant
             \fI(public member function)\fP
